1. Field of the Invention
The present invention relates in general to production of integrated circuit packages and, more particularly, to method and device for molding integrated circuit packages using lead frames free from a dambar.
2. Description of the Prior Art
In recent times, cheap IC packages (integrated circuit packages) such as made of synthetic resins have been generally used rather than expensive IC packages such as made of ceramic, thus to cut down expenses in using the IC packages. The quality improvement of the resin IC packages makes it possible to produce high value added IC packages using cheap resins. In the past, such high value added IC packages would be produced only using expensive ceramic.
The cost of IC packages is increased due to expense for trimming equipment as well as to waste of labor and time for trimming the package assembly, which package assembly is a unit product in the package state. The trimming operation for the package assembly also may cause the products to be delivered after delay of the time limit for delivery and this gives a bad effect on production and sale of the IC packages.
In the near future, the lead interval of an IC package will be reduced from at least 0.35 mm to about 0.15 mm, so that the expenses for punches, which punches are used for cutting off lead connecting dambars from the lead frame, as well as for the trimming equipments will be very heavy.
Moreover, the dambar cutting operation for the package should give a bad effect on the leads and this may cause deformation and damage of the leads due to shock generated in the trimming step and in the forming step. Thus, high quality products may be not provided.
In this regard, an IC package is needed which is produced by forming a molded housing around a lead frame free from a dambar (hereinbelow, referred to simply as "the dambarless lead frame").
The procedure for production of a typical IC package is started with inspection of raw material for the package as represented in the flow diagram of FIG. 1. The raw material inspection step is followed by a die preparation step, otherwise stated, a chip preparation step. Thereafter, the package production procedure is carried out in order of a die (chip) bonding step, a wire bonding step, a molding step (a packaging step), a trimming step, a plating step, a marking step and a forming step.
Both the molding step and the trimming step of the above procedure will be described in detail hereinbelow.
Molding (packaging) step: the semiconductor chip interconnecting to the lead frame by means of wires is molded in predetermined shape of mold dies using thermosetting resins, thus to form a molded housing. The molded housing, which is formed around the lead frame, is to protect the internal circuit devices and the semiconductor chip from external impact as well as to form the configuration of the package. In the molding step, the thermosetting resins are hardened for a given time and at a given temperature, thus to provide desired rigidity for the molded housing.
Trimming step: the dambars, which connect the leads to each other so as to prevent possible resin leakage during the molding step, are cut off in the trimming step from the lead frame of the package after the molding step so that the configuration of each resulting package agrees with use of the package.
In order to mold the package 15, a lead frame shown in FIGS. 4 and 5 is laid on a bottom mold die BM shown in FIG. 6. Thereafter, a top mold die TM is set on the bottom mold die BM so that the top mold die TM presses the dambars DB of the lead frame LF (FIG. 5). In this state, the thermosetting resins are injected into a cavity defined between the top and bottom mold dies TM and BM so as to form the molded housing around the lead frame LF. In this molding step, the dambars DB connecting the leads L of the lead frame LF to each other prevent the resins from leaking out of the two mold dies through the gaps between the leads L.
As shown in FIGS. 8, 9 and 10, each of the top and bottom mold dies TM and BM has dambar pressing projections P, each projection P having a flat pressing surface. When molding a package using a dambarless lead frame LF, the resins may leak either from the gaps between the leads L or from the gaps between the top and bottom surfaces of the lead frame LF and the top and bottom mold dies TM and BM even though the leads L of the dambarless lead frame LF are tightly pressed by the mold dies TM and BM. This gives a bad effect on the package molding conditions and deteriorates quality of products. In this regard, it is nearly impossible to mold and produce IC packages using the dambarless lead frames in the prior art. The package molding part 11 and the part 12 is outside the package.